FIG. 1 shows a portion of a typical electrical power distribution network 10. The network comprises a number of branches connecting nodes indexed 00 to 08. A plurality of power sources A to D are connected to nodes 03, 05, 07 and 08 respectively and demand is drawn from the network at the nodes indicated with the arrows 12. The network 10, in this case, is fed through two transformers 14 connected in parallel. In the example shown, the power sources are indicated as being wind turbines, although it will be appreciated that the present invention is concerned with mitigating problems using any form of power resource capable of reactive power control including wind, photovoltaic, and hydro sources. Also, while the power sources are shown as individual turbines, these can equally be wind farms including a plurality of turbines or any such group of generators.
It will be appreciated that many such power sources are connected to remote portions of distribution networks where the capacity of the infrastructure connecting power sources to demand may be limited. The physical properties of the conductors and loads within the network cause both voltage magnitude and angle to vary and so cause the flow of reactive power. Any reactive source or sink can reduce the active power transfer capacity of network branches and could lead to congested power flows in the network.
The power available from sources A to D can vary largely according to prevailing environmental conditions to the extent that, under certain operating conditions, providers can be asked by network operators to curtail active power generation to avoid network congestion and breaching thermal constraints i.e. current limits for the network.
In short, there are two conflicting constraints in play, maintaining the terminal voltage of a generator at an acceptable level (to the generator) to allow generated power to be delivered and monetized while ensuring thermal constraints are not breached.
Current management schemes for generators on such distribution systems have the potential to cause the underutilisation of existing network infrastructure, where in attempting to maintain a balanced voltage at a generator's connection node (and the surrounding network) the required reactive power may limit the available transfer capacity for active power on the surrounding lines. This in turn results in the unnecessary curtailment of active power and loss in revenue for those generators.
There have been a number of attempts to address problems with power distribution in such networks:
WO2009083445 discloses a method for increasing a total reactive power capability of a cluster of wind turbines operationally connected to a grid. The method comprises the steps of generating a voltage value representative of a grid voltage level, determining a total required reactive power value based on the voltage value, and activating at least one wind turbine in said cluster to increase the total reactive power capability from a present value to the required total reactive power value by a predetermined amount.
U.S. Pat. No. 7,531,911 and EP2254217 disclose a method of reactive power regulation in a wind farm having a plurality of wind turbines that provides optimum control of the reactive power compensation in the wind farm and is able to keep a reactive power reserve in order to support eventual grid contingencies.
U.S. Pat. No. 6,924,565 provides real and reactive power control for wind turbine generator systems where the reactive power support provided by individual wind turbine generators in a system can be dynamically varied to suit application parameters and so utilize the total capacity of a wind turbine generator system.
U.S. Pat. No. 6,924,627 discloses reactive power regulation in a wind power installation modulated by a compensation device between a generator and the network for the compensation of reactive power. The compensation device is so regulated that the electrical power delivered to the consumer has a reactive power component which is adapted in respect of its phase, amplitude and/or frequency to the consumer in such a way as to compensate for the reactive power in the consumer.